Estimation of Passenger Car Equivalents Based on Capacity Variability

Abstract

Passenger car equivalents are used in highway capacity analysis procedures to convert mixed traffic stream volumes into equivalent passenger car volumes. This paper presents a new empirical method for estimating passenger car equivalents for heavy vehicles on freeways. The proposed approach is based on the concept of stochastic capacities. Capacity distribution functions are estimated in passenger car units. With a determination of the equivalency factor for which the coefficient of variation of the capacity distribution function becomes minimal, passenger car equivalents for heavy vehicles can be derived. The methodology is demonstrated for a number of freeway bottlenecks. The new approach also gives insight into the extent at which the variance of prebreakdown volumes is attributable to variable percentages of heavy vehicles.

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